The engine of a motor vehicle is supported on flexible engine mounts and can rock during operation. Because of the mass of the exhaust pipe and the manner of its mounting on the vehicle body, rocking of the engine gives rise to significant stresses in the exhaust system which reduce the life of the system.
In order to assist in isolating the exhaust system from the rocking of the engine, flexible ball joints have been proposed for use at the junction between two pipe sections. The term "flexible ball joint" is used in the present specification to a refer to a joint having a collar with a part-spherical surface which seals between a flange on one of the pipe sections and a part-spherical seat on the other, the joint allowing the pipe sections to pivot relative to one another without any escape of exhaust gases.
Such a joint has been used at the lower end of the down pipe, but there it is far removed from the rocking axis of the engine. The length of the down pipe exaggerates the rocking and introduces considerable side to side movement, which cannot be accommodated by a single flexible ball joint.
It has also been proposed to use a flexible ball joint at the top end of the down pipe, where it joins on to the exhaust manifold. In this case, the down pipe has but a single conduit and it is not possible to tune the length of the branches of the exhaust manifold to enhance scavenging.
GB-A-1 586 244 describes an exhaust system for a motor vehicle internal combustion engine, in which the lower end of the exhaust manifold and the upper end of the down pipe each comprise two conduits which are sector shaped in cross section and are arranged to form a complete circle split by radial walls. The circumference of the down pipe is sealed by a flexible ball joint against the circumference of the manifold and the radial walls of the down pipe are spaced from the radial walls of the manifold by a small gap sufficient to permit pivoting of the down pipe relative to the manifold.
Though this exhaust system is capable of permitting the engine to rock relative to the down pipe, the construction and design of the clamping bolts which hold the parts of the flexible ball joint against one another interfere with the operation of the ball joint and are a source of squeaks.